Few of us really appreciate how hardworking and integral the
rear derailleur is until it stops functioning perfectly. Then you’re stuck with, at best, an imprecise and clattery ride. So the next time you
climb with little—well, a little less—effort, give a nod to this marvel of engineering. Here’s how it works—plus tips to keep it running like new.
1. The upper area on the derailleur is the b-knuckle.
2. On Shimano and SRAM derailleurs, the b-tension adjuster fine-tunes the height of the guide pulley. On
Campagnolo derailleurs, the adjustment is done via a screw near the p-knuckle.
3. To adjust cable tension, use the barrel adjuster.
4. The guide pulley keeps the chain in line as it moves from cog to cog during shifts.
5. The idler pulley helps hold tension on the chain regardless of gear choice.
6. The mounting bolt connects the derailleur to the frame.
7. The parallelogram linkage lets the chain move left and right and up and down the cassette while remaining parallel to the cogs.
8. High and low limit stops are usually found near the b-knuckle but sometimes on the front of the parallelogram. The low limit (often marked L) prevents shifts into the spokes; the high limit (marked H) prevents the chain from dropping off the smallest cog.
9. The p-knuckle holds both the guide pulley and a spring that keeps tension on the cage to hold the chain taut. Shimano’s XTR Shadow Plus (for mountain bikes) has a switch that increases spring tension and activates a friction stabilizer to keep the chain from bouncing in rough terrain.
10. The cable bolt pinches the shift cable in place. On most derailleurs, when the shift lever pulls on the cable, the derailleur moves up the cassette to a lower gear. The exception: Shimano’s low-normal derailleurs, on which it does the opposite.
11. The cage keeps the chain in line between the pulleys.
Clean and Adjust
For the best performance, position the guide pulley (4) roughly 6 to 7mm away from the cogs. If your chain skips when you shift to a lower gear, give the barrel adjuster (3) a half-turn counterclockwise (Tip: Think L for lower and for left turn). A higher gear? Try a halfturn clockwise. After cleaning, add a few drops of lube to the pivot points (there are eight) of the parallelogram linkage (7), cycle through a few shifts, then wipe off any excess lube. When you clean your chain, clear the muck from the inner and outer surfaces of both pulleys (4 & 5)—it collects at the base of their teeth.
Article From "Bicycling.com"
Other Related Articles and Pages:
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http://en.wikibooks.org/wiki/Bicycles/Maintenance_and_Repair/Derailleurs/Rear_Derailleur_Adjustment
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http://www.wikihow.com/Adjust-a-Rear-Bicycle-Derailleur
